To some people, social networking sites like Facebook and Twitter are nothing but a waste of time; a platform for narcissism, AND sharing facile observations with hundreds OR even thousands of online friends. Others however, regard social media more as a force for good, a tool for political and social change, as evidenced by the role played by facebook and twitter in mobilizing pro-democracy protestors in much of the Arab world this year. Dhiraj Murthy teaches sociology and anthropology at Bowdoin College in Brunswick, and is one of the world's leading authorities on social media. He's currently writing a book called "Twitter: Social Communication in the Twitter Age." He says an estimated 500 million people now regularly use Facebook, while the number of Twitter followers is not that far behind.

Dhiraj Murthy's book "Twitter: Social Communication in the Twitter Age" is due to be published next summer by Polity Press.

In the meantime he can be followed on Twitter: @dhirajmurthy.